International Symposium on Tsunami Disaster Mitigation in Future
Jan. 17-18, 2005, Kobe, Japan
From the flume results, normalized maximum vertical runup was plotted versus an energy-based
parameter using peak runup velocity. It suggests that a linear relationship can be used to predict
runup for a known horizontal runup velocity and that friction losses amount to 20 percent of the total
energy during runup.
Runup heights measured in the longshore direction along the beach showed very good uniformity
for different source lengths (i.e., DSWG lengths). The effect of eccentricity of the source on runup
was studied by varying the offset of the source from the measurement points on the beach. Runup
values were largest directly opposite the center of the source and decreased linearly with longshore
distance due to diffraction. Runup showed a strong linear trend with source length, increasing as the
source length increased. The final results from the plane beach experiments illustrate the evolution of
maximum amplitude with cross-shore distance in the basin. Test results show that dimensionless
wave height increases as source length increases and water depth decreases, in agreement with earlier
findings of Synolakis (1991) relative to Green's Law. This is probably the first instance where it has
been proven that this evolution law is valid for 3D waves.
Vertical wall
The second set of experiments included a flume study of tsunami runup on a vertical wall to study the
effect of compound bathymetry on this highly nonlinear phenomenon (Briggs et al. 1996b). The
compound-slope, fixed-bed bathymetry consisted of three different slopes (1:53, 1:150, and 1:13) and
a flat section in the deep end. Figure 2a is a schematic of the flume setup and 2b is a photograph of
Drs. Synolakis and Briggs observing the runup. The vertical wall was located at the landward end of
the 1:13 slope. The water depth in the flat section of the flume measured 21.8 cm.
Ten capacitance wave gages were used to measure surface wave elevations along the centerline of
the flume. Three target wave heights H=0.05, 0.30, and 0.70 were simulated for Cases A, B, and C,
respectively. When the waves reached the vertical wall, a plume of water would shoot upward.
Wave breaking occurred for Cases B and C only. For Case B the wave broke at or near the wall.
For Case C the wave broke between gages 7 and 8 (i.e. in front of the toe between the 1:13 and 1:150
slopes) before re-forming and shoaling to the vertical wall. The largest runup at each depth was
recorded for Case B, which experienced wave breaking only at or near the wall.
Circular island
The third series of experiments involved a physical model of a circular island (Briggs et al. 1994,
1995b, 1996a, and Liu et al. 1995). This study was motivated by the 1992 tsunami off Flores Island,
Indonesia, which killed nearly 2,500 people (Yeh, et al. 1994). Reflections off Flores Island may
have been partially responsible for the tsunami waves that completely destroyed two villages on the
adjacent Babi Island, in sheltered areas on the lee side of the island. Because Babi Island is nearly
circular in shape, a laboratory experiment was deemed necessary to better understand the complex
physics involved in why the tsunami wave split in two and traveled around both sides of the island
before reforming on the lee side and producing the unexpected destruction.
The model island was constructed in the center of a 30-m-wide by 25-m-long flat-bottom basin
(Figures 3a and 3b). The island had the shape of a truncated, right circular cone with diameters of 7.2
m at the toe and 2.2 m at the crest. The vertical height of the island was approximately 62.5 cm, with
1 vertical on 4 horizontal beach face. The water depth was set at 32 cm in the basin. Twenty-seven
capacitance wave gages were used to measure surface wave elevations. The first four gages were
located parallel to the wavemaker to measure incident wave conditions. A measurement grid of six
concentric circles covered the island to a distance 2.5 m beyond the toe.
Measurement points were
located at the intersection of these concentric circles and the 90-deg radial lines. The spacing
between grid points was a function of the water depth.
The DSWG was used to generate solitary waves. The full length of the DSWG was used to
generate three solitary wave cases. Target normalized wave heights of H=0.05, 0.10, and 0.20 were
simulated for Cases A, B, and C, respectively. All waves were non-breaking until final stages of
transformation near the shoreline (where gentle spilling occurred) except for the Case C wave, which
broke nearshore.
Maximum vertical runup was measured at twenty locations around the perimeter of the island.
Sixteen locations were evenly spaced every 22.5 deg around the perimeter. Four radial transects with
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